Rapid depletion of soil organic carbon stock threatens agricultural sustainability in Iran
Mehdi NOURZADEH HADAD1,*(), Jose Antonio RODRIGUEZ MARTIN2, Akbar HASSANI3
1Nuclear Agriculture Research School, Nuclear Science and Technology Research Institute (NSTRI), Karaj 3148643111, Iran 2Department of the Environment, National Institute for Agricultural and Food Research and Technology (INIA-CSIC), Madrid 28040, Spain 3Soil Science Department, University of Zanjan, Zanjan 45371-38791, Iran
Soil organic carbon (SOC) stocks play a critical role in maintaining soil fertility, regulating biogeochemical cycles, and mitigating climate change, yet they are increasingly threatened in semi-arid agricultural regions. This study quantified spatiotemporal changes in SOC concentration and stocks across Hamadan Province, western Iran, over a 16-a period (2010-2025), a region that has undergone rapid agricultural intensification. A total of 212 soil samples were collected from the same georeferenced locations during both sampling campaigns, targeting the 0-30 cm soil layer. SOC concentration was determined using standardized laboratory methods, and SOC stock was calculated by integrating SOC concentration, bulk density, soil depth, and coarse fragment content. Geostatistical analyses, including experimental semivariograms and Ordinary Kriging, were applied to characterize spatial variability and to generate continuous maps of SOC concentration and SOC stock for both years (2010 and 2025). Results indicated a marked decline in mean SOC concentration, from 0.99% in 2010 to 0.85% in 2025. Correspondingly, mean SOC stock decreased from 126.48 to 84.45 Mg C/hm2, representing a reduction of 33.23% and a total carbon loss of approximately 4.59 Tg C from agricultural topsoil across the province. Spatial analysis revealed increased micro-scale variability and extended spatial dependence over time, reflecting increasingly heterogeneous management effects superimposed on broader landscape controls. Severe SOC stock depletion (SOC stock ratio (the ratio of SOC stock in 2025 to that in 2010) <0.50) occurred as scattered patches across the province, while moderate declines (0.50-1.00) dominated most agricultural areas, and some northern and southern zones showed comparatively greater stability or localized carbon gains. This study provides the first long-term, spatially explicit assessment of SOC stock dynamics in western Iran, demonstrating rapid carbon depletion driven by intensive cultivation under semi-arid conditions. The findings highlight the urgent need for soil conservation and carbon-preserving management practices to sustain soil health, agricultural productivity, and climate mitigation potential in dryland farming systems.
Received: 30 December 2025
Published: 31 July 2026
Mehdi NOURZADEH HADAD: conceptualization, investigation, resources, funding acquisition, supervision, and writing - original draft preparation; Jose Antonio RODRIGUEZ MARTIN: conceptualization, investigation, methodology, formal analysis, validation, and writing - review and editing; Akbar HASSANI: investigation, supervision, funding acquisition, data curation, and writing - original draft preparation. All authors approved the manuscript.
Mehdi NOURZADEH HADAD, Jose Antonio RODRIGUEZ MARTIN, Akbar HASSANI. Rapid depletion of soil organic carbon stock threatens agricultural sustainability in Iran. Journal of Arid Land, 2026, 18(7): 1179-1191.
Fig. 1Spatial distribution of soil sampling sites across administrative agricultural lands in Hamadan Province
Soil property
Year
Mean
Median
Minimum
Maximum
SD
BD (g/cm3)
2010
1.42
1.41
1.12
1.68
0.11
2025
1.48
1.47
1.18
1.75
0.12
SOC concentration (%)
2010
0.99
0.92
0.02
3.40
0.50
2025
0.85
0.79
0.00
3.31
0.50
Sand (%)
2010
33.50
32.00
17.00
76.00
13.80
2025
33.66
32.00
18.00
75.00
14.00
Silt (%)
2010
38.60
38.00
12.00
61.00
9.50
2025
38.10
38.00
12.00
62.00
9.40
Clay (%)
2010
28.00
30.00
9.00
48.00
7.60
2025
28.60
29.40
8.00
48.00
7.50
NO3- (mg/kg)
2010
13.09
9.59
1.32
96.70
11.98
2025
17.20
12.24
1.98
128.93
15.69
NH4+ (mg/kg)
2010
1.58
1.32
0.00
7.05
1.18
2025
2.11
1.76
0.11
9.05
1.50
pH
2010
7.59
7.60
6.50
8.30
0.23
2025
8.07
8.09
6.93
8.85
0.25
EC (dS/m)
2010
1.23
0.87
0.34
10.20
1.23
2025
1.64
1.13
0.41
13.60
1.66
AP (mg/kg)
2010
11.90
8.48
1.60
75.60
11.27
2025
15.03
10.60
1.92
90.72
14.35
EK (mg/kg)
2010
327.16
290.00
70.00
1000.00
169.00
2025
425.44
378.67
93.33
1250.00
216.72
Table 1 Statistical summary of descriptive soil properties in 2010 and 2025
Year
SOC stock (Mg C/hm2)
Amount of carbon (Tg C)
Mean
Minimum
Maximum
SD
2010
126.48
17.35
400.51
12.15
34.41
2025
84.45
4.05
328.01
9.63
29.82
Change (from 2010 to 2025)
-42.03
-13.30
-72.50
-4.59
Table 2 SOC stock at the 0-30 cm depth in 2010 and 2025
Fig. 2Experimental and model variograms for soil organic carbon (SOC) concentration in 2010 (a) and 2025 (b). Notable changes in spatial structure parameters were observed between the two sampling years.
Year
Mean prediction error (%)
Average standard error (%)
RMSE (%)
Standardized RMSE
2010
-0.000532
1.486
1.554
1.008
2025
0.000203
1.477
1.504
1.024
Table 3 Cross-validation indices for Ordinary Kriging interpolation of SOC concentration
Fig. 3Spatial distribution of SOC concentration across Hamadan Province in 2010 (a) and 2025 (b)
Fig. 4Spatial distribution of SOC stock across Hamadan Province in 2010 (a) and 2025 (b)
Fig. 5Spatial distribution of SOC stock ratio (the ratio of SOC stock in 2025 to that in 2010) across Hamadan Province. Values <1.00 indicate SOC stock decline.
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